1. Field of the Invention
The present invention relates to a semiconductor laser, particularly to a semiconductor laser used for a light source for reading optical information and a module for optical communication.
2. Description of the Related Art
An edge-emitting semiconductor laser comprises n-type and p-type cladding layers with a low refractive index and an active layer with a high refractive index formed between the cladding layers so as to emit light from the edge of the active layer. The energy band gap of the cladding layers is larger than that of the active layer.
Various structures are proposed to control the transverse mode of the semiconductor laser. There are the flection waveguide type in which an active layer is terraced and the rib type in which a region along a waveguide of an active layer is thickened.
There are many various structures of the flection waveguide, and some of them are described as follows.
FIG. 1 is a perspective view showing the first embodiment of a semiconductor laser according to the prior art which is described in Japanese unexamined publication (KOKAI) Hei 2-77124.
This semiconductor laser comprises an n-GaAs substrate 101 having a groove 102 formed on the (001) plane of the n-GaAs substrate 101; an n-GaAs buffer layer 103, an n-AlGaInP cladding layer 104, an InGaP active layer 105, a p-AlGaInP cladding layer 106, a p-InGaP layer 107, and an n-GaAs current constriction layer 108 formed in order on the n-GaAs substrate 101; and a contact layer 109 in which Zn is diffused in the current constriction layer 108 above the groove 101.
In the case of the above constitution, the cladding layers 104 and 106 and the active layer 105 are also flexed according to the concave and convex of the groove 102 and a striped pumped region (gain region) is formed on the active layer 105 put between two side slopes. Therefore, a laser beam emitted from the edge of the pumped region has a small astigmatism and a preferable characteristic is realized.
FIG. 2 is a front view showing the second embodiment of a semiconductor laser according to the prior art which is described in Japanese unexamined publication (KOKAI) Hei 5-82892.
For this semiconductor laser, the structure of the layers from the n-GaAs substrate 101 to the active layer 105 is the same as that in FIG. 1. However, the structure is different from that in FIG. 1 in that a p-AlGaInP cladding layer 110 formed on the active layer 105 protrudes like a mesa along a waveguide, a p-GaAs layer 112 is further formed on the protruded portion of the cladding layer 110 through a p-InGaP layer 111, and a current constriction layer 114 made of n-GaAs is formed at the both sides of a convex portion 113 produced to these layers 110, 111 and 112, and moreover a p-GaAs contact layer 115 is formed on the current constriction layer 114 and the convex portion 113.
This type of device realizes a low-threshold and high-efficient laser operation, because the current constriction layer 114 approaches the vicinity of the active layer 105 along the wave guide.
FIG. 3 is a perspective view showing the third related art of a semiconductor laser which is described in Japanese application of No. Hei 5-58288 but not disclosed. One of the inventors of the application is an inventor of the present invention.
This semiconductor laser comprises the GaAs substrate 101 having a groove 117, and the groove 117 near by a light outputting edge is narrower than the opposite edge by changing the width of the groove 117 by stages or continuously. Thereby it is possible to decrease excess noises induced by optical feedback because the mode volume of the laser increases and multiple-mode oscillation is performed.
In FIG. 3, laminated structures other than the above are the same as those of the semiconductor laser shown in FIG. 2 and the same symbol shows the same element. Therefore, the description of them is omitted in this case.
FIG. 4 is a perspective view showing the fourth related art of a semiconductor laser which is described in Japanese application of No. Hei 4-250286 but not disclosed. One of the inventors of the application is an inventor of the present invention.
This semiconductor laser is constituted by forming a different composition of semiconductor layer on a main surface of an n-GaAs substrate 120 having a plane being 6.degree. off to the &lt;110&gt; direction (direction A) from the (001) plane. There is a terrace on the main surface of the n-GaAs substrate 120, the difference in level of the terrace is extended along the &lt;1 10&gt; direction and a slope 121 tilted toward the direction A by approx. 20.degree. is formed on the difference in level. On the n-GaAs substrate 120, an n-(Al.sub.0.7 Ga.sub.0.3)InP cladding layer 122, an InGaP active layer 123, a p-(Al.sub.0.7 Ga.sub.0.3)InP cladding layer 124, a Zn--Se simultaneous-doping AlGaInP layer 125, a p-(A.sub.10.7 Ga.sub.0.3)InP layer 126, a p-InGaP layer 127, and a p-GaAs contact layer 128 are formed in order on the main surface.
The slope 121 has the A-plane into which a VI-group dopant is more easily incorporated than a II-group dopant when film is formed by means of MOVPE. By doping II- and VI-group elements in the time of forming a semiconductor film, only a semiconductor layer on the slope 121 is transformed into the p-type and other flat regions are transformed into the n-type. The AlGaInP layer 125 is formed by this Zn--Se simultaneous-doping method. Thus, a p-type layer is formed on the slope of the active layer 123 and an n-type layer is formed on other regions. As a result, a pnp current constriction structure is formed at the both sides of the slope 121 and the active layer 123 along the slope 121 serves as a pumped region.
Though not illustrated, an electrode is formed on the bottom of reach substrate and the top of the contact layer.
Because each of the above active layers has a small energy band gap and its edge is brought under a high light-density state when a laser beam is emitted, light absorption due to non-radiative recombination occurs at the edge. When the output of a semiconductor laser increases, the light absorption of the active layer increases, and the temperature rises at the edge. Therefore, the energy band gap is further decreased due to the temperature rise and the light absorption value further increases. Thus, a COD (Catastrophic Optical Mirror Damage) breakdown occurs. The COD breakdown occurs at an output of approx. 60 mW in each semiconductor laser described above.
In this case, it is considered to decrease the light density at the edge of an active layer and to increase the output by widening the stripe width of the active layer. However, this structure is not proper because a laser beam does not greatly diverge in the direction of the stripe width of the active layer, the aspect ratio of emitted light increases, and the shape of the laser beam becomes far from a circle which is requested for application of equipment.